well for $15k, this should do the trick
These are my favorite cars in that range, very powerful, very classy, very clean and yet they have that retro feel! If you can find a good one that had a good history, Id get it all day!
Forget the A4, they're slow, any S4's from the mid 2ks with mid-high miles will be in your range
Benz C300s can be had any they are sexyyy but not too quick
High miles 335i could be had but they've probably been beaten up to be in your range, still hold pretty good value.
Also to check out in ricer/sport sector are:
Supra MKIV (before the 4 they were ehhh)
3000GT VR-4/Stealth TT
Nissan GTR R32/33/34 (if you can find one
R34's will run more, all depends on condition though as well
C5 Vette Z06 - can be had for $20k and down to $15 if you have a good eye
CTSV - older gen like already mentioned (nice car, similar to the M5 overall)
Hope that gives you some food for thought, g/l with your quest buying a new car is always a ton of fun